The present invention relates to a vehicle braking system with a hydraulically controllable disc-brake device which exhibits an electromechanical actuating device for activating a parking-brake function, the vehicle braking system exhibiting a hydraulic circuit with a hydraulic pressure source and also with controllable hydraulic functional elements, in order to control the disc-brake device hydraulically in accordance with a service-braking action of a driver or in accordance with an automatic activation, the vehicle braking system further exhibiting a control device, in order to control the electromechanical actuating device in accordance with a parking-brake action of the driver or in accordance with an automatic activation of the parking-brake function. The invention further relates to a method for controlling a vehicle braking system of the type described above.
Vehicle braking systems are known from the state of the art in diverse forms. In concrete terms, document DE 10 2005 051 082 A1, and corresponding U.S. Pat. No. 8,322,495 B2, both of which are incorporated by reference herein in entirety, describes a vehicle braking system with a disc brake that has been constructed as a floating-calliper disc brake. In this case, a friction-lining arrangement with two opposing friction linings has been provided on both sides of a brake disc. The one brake lining has been fitted to a floating calliper, whereas the other brake lining can be displaced hydraulically relative to the floating calliper via an actuating piston. As a result of a hydraulic pressurisation of an actuating piston, the movable brake lining is displaced towards the brake disc. The brake lining that is fixed to the floating calliper is pulled from the other side against the brake disc in known manner with the aid of the floating-calliper function, so that a braking force acts on the brake disc from both sides. But, in addition to this floating-calliper function which has long been known, this state of the art also provides an electromechanical actuating device. The latter is used in order to realise a parking-brake function. Given appropriate control, in addition to the hydraulic displacement it is also possible for the disc brake to be actuated electromechanically. But it has been shown that in the case of such a combination of a hydraulically and electromechanically actuatable brake residual slippage torques may remain in the system after the disengagement procedure as a result of an electromechanical actuation that has taken place previously, which torques have to be suppressed by means of elaborate separate measures. This is due to the fact that the conventional ‘rollback function’ of a disc-brake arrangement of such a type, which prevents residual slippage torques after a hydraulic actuation, cannot be fully exhausted in the case of the electromechanical actuation, on account of the lack of application of hydraulic pressure. By the term ‘rollback function’, one understands the effect of an elastically deformed hydraulic seal which in the housing is ordinarily received in a receiving groove and which by means of its inner surface is in adhering contact with the surface of the actuating piston, whereby this adhering contact firstly prevents a slipping of the hydraulic seal on the surface of the actuating piston during an actuating movement relative to the housing. As a result, the hydraulic seal which is retained in the housing of the receiving groove there undergoes in the course of this movement an elastic deformation which after reduction of the actuating force can elastically relax back into its initial state and in the process pulls the brake piston back into an initial position which is largely ineffectual in terms of braking action. The deformability of the hydraulic seal—that is to say, its capacity to follow a movement of the actuating piston by means of an elastic deformation—is decisively limited by the holding forces determining the adhering contact. In the course of the elastic deformation this means that as soon as the constantly rising forces, counteracting the movement of the actuating piston, exceed the holding forces of the adhering contact the hydraulic seal slides on the surface of the actuating piston. The region of application of the hydraulic seal on the actuating piston changes as a result, and the actuating piston slides through under the hydraulic seal. It is furthermore to be taken into consideration that the pressure fed in during a hydraulic actuation increases the normal force with which the hydraulic seal is pressed against the surface of the actuating piston, by reason of the elastic deformation of the hydraulic seal that is caused thereby, as a result of which the holding forces of the adhering contact increase to the same degree in comparison with an electromechanical actuation without supply of a hydraulic pressure. The lack of pressure-induced deformation and the correspondingly smaller holding forces may in the case of an electromechanical actuation have the result that in the case of an equally large movement of the actuating piston the adhering contact breaks off before reaching the end position of the actuating piston, on account of the absence of the increase in normal force. As a result, in comparison with a hydraulic pressurisation the rollback function as described above cannot be fully exhausted. This may result in undesirable residual slippage torques.